Methods of making electric heating units of the sheathed resistance conductor type



T. MCAVOY y ETAL METHODS OF MAKING ELECTRIC HEATING UNITS OF' 2 Sheets-Sheet l Filed Oct. 5, 1960 INVENTORS Thomas McAvoy Eugene F 7i/lan Ev m Saga@ wm l May 4, 1965 T. MCAVOY ETAL METHODS OF MAKING ELECTRIC HEATING UNITS OF THE SHEATHED RESISTANCE CONDUCTOR TYPE 2 Sheets-Sheet 2 Filed Oct. 5, 1960 INVENToRs Thomas McAvoy Eugene l.: Dil/on fF Anya.

3,181,230 METHDS Old MAKING ELECTRIC HEATENG UNliTS OF THE SHEATHED RESISTANCE CON- DUCTGR TYPE Thomas McAvoy, Roselle, and Eugene F. Dillon, Chicago,

lil., assignors to General Electric Company, a corporation of New York Filed Oct. 3, 1960, Ser. No. 60,104 7 Claims. (Cl. 29-155.63)

The present invention relates to methods of making electric heating units, and more particularly to such methods of filling with granular refractory material the sheaths of :such heating units of the sheathed resistance conductor type.

Electric heating units of the sheathed resistance conductor type are ordinarily manufactured by the method, and employing the loading machine, as disclosed in U.S. Patent No. 2,316,659, granted on April 13, 1943 to John L. Andrews. In this manufacturing method, an assembly is produced of the elongated helical electric resistance conductor and the pair of electric terminals respectively electrically connected to the opposite ends of the resistance conductor and the elongated tubular metallic sheath surrounding the resistance conductor and at least the inner ends of the terminals in spaced relation therewith and with the top end of the sheath in open condition and with the bottom end of the sheath in closed condition. This loading machine is then operated, whereby a iirst charge of the granular refractory material (ordinarily crystalline magnesium oxide) is introduced into the open top end of the sheath of the supported assembly and falls by gravity into the bottom end thereof, and then the tubular hammer is inserted into the open top end of the sheath and is dropped downwardly therein, in surrounding relation with the resistance conductor, to tamp in place the lirst charge of granular refractory material mentioned. This loading machine is further operated, whereby the hammer is raised and withdrawn from the open top end of the sheath, and a second charge of the granular refractory material is introduced into the open top end of the sheath, and then the hammer is again inserted into the open top end of the sheath and is again dropped downwardly therein to tamp into place the second charge of granular refractory material mentioned. In the operation of this loading machine, the above-described steps are repeated a great number of times and until the sheath is filled with the granular refractory material, so as to embed therein the resistance conductor and the inner ends of the terminals and to retain the same in place in spaced relation with the sheath.

ln this manufacturing method, the loaded assembly is then removed from this loading machine, and the upper end of the sheath thereof is stoppered, whereupon the assembly is transferred to a rolling machine, such, for example, as that disclosed in US. Patent No. 2,677,172, granted on May 4, 1954 to Sterling A. Oakley. This rolling machine is operated to reduce the diameter of the sheath of the assembly and thus highly to compact the granular refractory material contained therein so as to reduce the loading of granular refractory material to a dense mass of electrical-insulating and heat-conducting material. Thereafter, the assembly is further worked to produce the finished electric heating unit of the sheathed resistance conductor type.

While this manufacturing method is entirely satisfactory, the steps employed therein, particularly the steps involved in the loading machine described above, are more expensive to carry out and more time-consuming than are desirable in the making of electric heating units of the type described upon a mass-production basis.

Accordingly, it is the general object of the present in- "Fr ited States Patent O "ice vention to provide an improved method of making electric heating units of the sheathed resistance conductor type that are more economical to carry out than those conventionally employed.

Another object of the invention is to provide an improved method of loading with granular refractory material the sheath of an assembly for an electric heating unit of the sheathed resistance conductor type, wherein the sheath of the assembly is filled with the granular refractory material employing a blowing step in which the granular refractory material is suspended in a stream of conveying gas and is transported therein into the open top end of the sheath of the assembly, and in which the granular refractory material is separated from the stream of conveying gas and projected into the bottom of the sheath, with the escape of the thus stripped conveying gas from the open top of the sheath, whereby the granular refractory material thus blown into the sheath of the assembly embeds the resistance conductor and the inner ends of the terminal retaining the same in spaced relation with the sheath.

Another object of the invention is to provide an improved method of loading the sheath of an assembly for an electric heating unit of the sheathed resistance conductor type, wherein air is first purged from the assembly employing chemically inert gas, and wherein the granular refractory material is then blow into the sheath of the assembly into embedding relation with the resistance conductor and the inner ends of the electric terminals connected to the opposite ends thereof utilizing a conveying stream of the chemically inert gas mentioned.

A still further object of the invention is to provide a method of the character described, comprising improved steps of effecting in a simple and ready manner, the filling with granular refractory material of the sheaths of assemblies for electric heating units of the sheathed resistance conductor type.

Further features of the invention pertain to the particular arrangement of the steps of the method, whereby the above-outlined and additional operating features thereof are attained.

The invention, both as to its organization and method of operation, together with further objects and advantages thereof, will best be understood by reference to the following specication, taken in connection with the accompanying drawings, in which: Y

FIGURE 1 is a diagrammatic illustration of a machine for filling the sheath of an assembly for an electric heating unit of the sheathed resistance conductor type and that may be employed in carrying out the steps of the present method of filling of the sheath of the assembly mentioned;

FIG. 2 is an enlarged vertical sectional view of the device for supporting the assembly for the electric heating unit during the filling of the sheath of the assembly, and forming a part of the machine of FIG. l; and

FIG. 3 is an enlarged vertical sectional view of a modified form of the device, as shown in FIG. 2.

Referring now to FIG. l, the machine lil' there illustrated is arranged to carry out the steps of the method of the present invention; and this machine l@ essentially comprises an upstanding substantially cylindrical vessel l1 provided with a substantially horizontally disposed diffusion member l2 dividing the interior thereof into upper and lower chambers 13 and ldrespectively disposed above and below the diffusion member l2. The eXtreme upper end of the vessel 11 terminates in a substantially frusto-conical section l5 that is connected to an upwardly extending conduit i6, and the extreme lower end of the vessel il terminates in a substantially frustoconical section 17 that is connected to a downwardly extending conduit 1S. The conduits 16 and 18 comprise a portion of a conduit system further including an upper substantially horizontally disposed conduit 19 in which there is arranged a particle or solids trap 20, as well as a vent pipe 21 communicating with the exterior and including a manually operable valve 22. The conduit system further comprises a fan 23 provided with an inlet fixture 24 connected to the adjacent end of the conduit 19 and an outlet fixture 25 connected to the adjacent end of an upstanding conduit 26. Further, the fan 23 comprises a rotor, not shown, carried by an operating shaft 27 that is driven by a suitable electric motor 2S. A condensate trap 29 is formed in the extreme lower end of the conduit 26 that is connected to the exterior by a drain conduit 30 including1 a manually operable valve 31. The extreme lower ends of the conduits 26 and 18 are connected together by a substantially horizontally disposed lower conduit 32, thereby completing the principal circuit of the conduit system.

The diffusion member 12 is adapted to support a fluidized bed of granular refractory material, indicated at 33, thereabove and in the upper chamber 13; and the conduit system mentioned is adapted to contain a body of chemically inert conveying gas having a relatively high molecular weight, such, for example as argon. For the purpose of supplying the conveying gas to the conduit system, a bottle or tube 34 containing the gas under high pressure is provided; which tube 34 is connected by a pipe 35 to a gas pressure regulator 36 that is operatively associated with the conduit 32, the pipe 35 including a manually operable valve 35a. When the valve 35a occupies its open position, the argon gas under high pressure in the tube 34 is throttled through the pipe 35 into the regulator 36; from which regulator 36 the argon gas is supplied to the conduit 32 dependent upon the pressure of the conveying gas in the conduit system. To this end, the casing of the regulator 36 houses valve mechanism, not shown, that is selectively governed by an associated expansible bellows 37 that controls the passage of the argon gas via a pipe 38 into the conduit 32, the bellows 37 being operatively connected via a pipe 39 to the conduit 32 for the pressure control purpose.

Further, the conduit system comprises a bypass conduit 4) extending between the conduits 19 and 32 and extending around the vessel 11 and including a manually operable valve 41. Also a drying arrangement for the conduit system is provided in the upstanding conduit 26 including a refrigerating machine; in turn, the refrigerating machine includes a compressor 42, a condenser 43, an expansion valve 44 and an evaporator 45, both the condenser 43 and the evaporator 45 being arranged in the conduit 26; and more particularly, the evaporator 45 is arranged above the condenser 43 and in spaced relation therewith in the conduit 25. Further, the conduit system comprises gauging and control facility, including two pressure gauges 46 and 47 respectively connected by two tubes 4S and 49 to the conduits 18 and 16 respectively adjacent to the frusto-conical sections 17 and 15 of the vessel 11.

Further, the machine comprises a hopper 5t? that is supported in upstanding position above the upper frustoconical section of the vessel 11 by a conduit 51 communicating between the bottom of the hopper 5u and the top of the upper chamber 13 through the upper truste-conical section 15 of the vessel 11. More particularly, the hopper 50 `comprises closed wall structure and is adapted to contain a body of the granular refractory material, as indicated at 52; and a rotary valve and -feed mechanism 53 is arranged jointly in the top of the conduit 51 and in the bottom of the hopper 5t). The rotary feed mechanism 53 is operative to supply or feed the granular refractory material 52 contained in the hopper 50 through the conduit 51 into the top of the upper chamber 13 through the upper frusto-conical section 15 of the vessel 11, without escape of substantial quantities of the argon gas therethrough from the vessel 11. The top of the hopper 50 is provided with a doorway that is closed by an inwardly opening top door, indicated at 54, the door 54 being biased by means, not shown, into its closed position in the doorway mentioned and readily accommodating the supply of granular refractory material from the outside into the hopper 5t) through the top doorway mentioned. Finally, the extreme top of the hopper 5G is connected to the exterior by a vent conduit 55 that includes a manually operable valve 56.

The granular refractory material 52 contained in the hopper 50, as well as the tluidized bed 53 of granular refractory material mentioned contained in the upper chamber 13 of the vessel 11, essentially comprises a finely divided material of the character of that normally employed in the manufacture of electric heating units of the sheathed resistance conductor type. Ordinarily, such material consists essentially of nely divided crystalline magnesium oxide, which material in highly compacted dense form constitutes an electrical-insulating and heatconducting material in the finished electric heating unit of the type mentioned.

Before proceeding further with the description of the construction and arrangement of the machine 1i), the general mode of operation thereof is here described.

In starting-up the machine 10, a suitable quantity 52 of the magnesium oxide is placed in the hopper 5) through the top door structure 54, and the feeding mechanism 53 is operated in order to cause a suitable quantity of the magnesium oxide to be discharged from the hopper 5t) through the conduit 51 onto the diffusion member 12. The valve 35a is opened so that the argon gas passes from the tube 34 through the pipe 35 into the regulator 36 and thence into the lower conduit 32, the regulator 35 being operative to maintain a predetermined gauge pressure of the gas in the conduit 32, which gauge pressure is disposed somewhat above atmospheric pressure. At this time, the argon gas being of relatively high molecular weight with respect to the normal light constituents of air, tends 4to accumulate in the lower portion of the conduit system, and specifically in the lower conduit 32. When the pressure of the gases stabilize in the conduit system, as indicated by the readings of the pressure gauges 45 and 47, the valve 22 in the vent pipe is opened; whereby air in the upper portion of the conduit system is discharged therefrom to the exterior. As time proceeds, the proportion of argon gas in the conduit system increases and utlimately substantially all of thc air is discharged or purged from the conduit system through the vent conduit 21 communicating with the upper conduit 19. Also, in the venting `or purging of the air from the conduit system, the valve 56 in the vent pipe 55 is opened, so as to allow the argon gas to displace air from the upper or vacant portion of the hopper 50. In the arrangement, the argon gas in the vessel 11 may pass through the conduit 51 and through the feeding mechanism 53 and thence through the charge 52 of magnesium oxide contained in the hopper 50, so that it may be purged from the upper portion of the hopper 50 to the exterior through the vent pipe 55 with the valve 56 in its open position. This arrangement, whereby a controlled leakage of the argon gas 11 from the vessel 11 through the conduit 51 and `through the feeding mechanism 53, is very advantageous by virtue of the fact that this argon gas sweeps through `the charge 52 of magnesium oxide contained in the hopper 52 sweeping occluded air from the granular refractory material mentioned and expelling the same to the exterior through the vent pipe 55. In the operation of the machine 10, it is recommended that the valve 56 be retained in its partially open position in order continuously to maintain the above-described sweeping action of the argon gas upon the charge 52 of the magnesium oxide contained in the hopper 50.

After substantially all of the air has been discharged asigao or purged from the conduit system by the admission of argon gas from the regulator 36 into the conduit 32, the vent pipe 21 is closed to the exterior by operation of the valve 22 back into its closed position. At this time, the motor 28 is operated so as to effect operation of the fan 23; whereby the argon gas in the conduit system is circulated from the fan 23 -through the conduits 26, 32 and 18 into the lower chamber or Windbox 14 of the vessel 11. From the lower chamber or windbox 14, the argon gas passes upwardly through the diffusion member 12 establishing the lluidized condition of the bed, as indicated at 33, in the upper chamber 13 of the vessel 11. Of course, the argon gas passing through the diffusion member 13 is capable of maintaining only a given head of the fluidized bed, as indicated by the substantially upwardly directed arcuate ycurve of the fluidized bed 33, so that the argon gas separates from the fluidized bed 33 proceeding upwardly into the upper frustoconical section 15 of the vessel 11 and thence via the conduits 16 and 19 baclr to the fan 23. Accordingly, the argon gas is circulated in a closed local circuit from the fan 23 through the conduit system described and upwardly through the vessel 11 so as to maintain the fluidized condition of the bed `33 in the upper chamber 13 of the vessel 11. Of course, the head of the fluidized bed 33 in the upper chamber 13 of the Vessel 11 is dependent upon the differential pressure across the diffusion member 12, as indicated by the pressure gauges 46 and 47; and for the purpose of pre-establishing the differential pressure mentioned, the valve 41 in the bypass conduit 40 is partially opened or preset so as to accommodate bypassing of at least a portion of the circulated argon gas from the lower conduit 32 directly into the upper conduit 19 and around the vessel 11. By proper setting of the valve 41 in the bypass conduit 4t), the required differential pressure may be maintained across the diffusion member 12 to obtain required fluidization of the bed 32 of crystalline magnesium oxide, without an undue head of the fluidized bed 32 into the upper portion of .the upper chamber 13 and without carrying of any substantial amount of the magnesium oxide with the argon gas into the conduit 16.

In passing, it is mentioned that it is highly undesirable for the granular magnesium oxide to be transported from the iuidized bed 33 into the conduit 16, since this matcrial is highly abrasive and thus causes substantial damage to the rotor of the fan 23 in the event any substantial amount of the granular magnesium oxide is entrained in the argon gas entering the casing of the fan 23. While the valve 41 in the bypass conduit 4@ is appropriately set so as to control the head of the fiuidized bed 33 and so as to prevent any substantial entrainment of the magnesium oxide in the argon gas passing from the truste-conical section 15 of the vessel 11 into the conduit 1o, the trap 2@ is included in the upper conduit 19 as a safety measure. Of course, the trap is operative or effective to remove any residual entrained small amount of magnesium oxide from the argon gas that is circulated from the upper conduit 19 back into the fan 23, thereby positively to prevent damage by abrasion to the rotor of the fan 23.

Also, at this time, the refrigerating machine is operated; whereby the compressor 42 discharges compressed gaseous refrigerant into the condenser 43 effecting liquifying of the compressed gas refrigerant in the condenser 43. The liquid refrigerant in the condenser 43 is expanded at the expansion valve 44 into the evaporator 45, and the vaporous refrigerant is returned from the evaporator 45 back to the compressor 42 to complete the cycle of the refrigerating machine. Accordingly, the evaporator 45 is cooled and the condenser 43 is heated by the operation of the refrigerating machine; whereby the circulation of the argon gas through the conduit 26 effects cooling thereof at the evaporator 45 and heating thereof at the condenser 43. The cooling of the circulated argon gas :at the evaporator 45 effects freezing of any entrained moisture therein with the accumulation of the moisture as frost upon the evaporator 45 and the consequent removal of moisture from, or drying of, the lcirculated argon gas in the conduit system. Of course, the circulated argon gas is again heated upon contact thereof with the condenser 43; whereby the overall effect of the operation of the refrigerating machine is slightly to heat the circulated argon gas, as well as to dry therefrom any moisture entrai-ned therein.

From time to time, it is necessary to shutdown the ma chine 10, whereupon the refrigerating machine may be shutdown, lso as to effect defrosting of 4the evaporator 45 and the consequent melting of the frost therefrom; whereupon the drippings from the defrosting evaporator 45 fall into the condensate trap 29. The condensate thus accumulated in the condensate trap 29 may be removed to the exterior through the drain pipe 30 by operation of the valve 31 into its open position dur-ing a short time interval, whereupon the pressure of the argon gas in the conduit system blows the condensate from the trap 29 through the drain pipe 30 to the exterior. Thereafter the valve 31 is returned back into its normal closed position.

vAs previously explained, during the normal operation of the machine l10, it is recommended that the valve 56 be retained in its partially open position so that there is a slow and controlled leakage of the argon gas from the vessel 11 through the charge 52 of magnesium oxide contained in the hopper 50` and thence via the ve-nt pipe 55 to the exterior, which arrangement is utilized tor the purpose of sweeping occluded air from the charge 52 of magnesium oxide contained in the hopper 5t). This arrangement insures that substantially no air is introduced into the vessel 11 incident to operation of the feeding mechanism 53 to supply :additional magnesium oxide from the charge 53 contained in the bed 50.

Continuing further with the construction and arrangement of the machine 10, the interior surface of the upper portion of the vessel .1v1 disposed above the diffusion member 12 is preferably provided with a protective coating or layer, indicated at 60, so as to prevent damage thereto by abrasion of the magnesium oxide in the fluidized bed 33. In the arrangement, the walls ofthe vessel 11 are normally formed oct steel, or other suitable metal, while the protective layer 60 may be formed of a suitable plastic material, such, for example, as a coating of latex, or the like, that is not substantially abraded by the magnesium oxide in the fluidized bed 33 and that can be readi-ly replaced, when required, as by a coating operation. Also an annular series of permanent magnets 61 are contacted by the magnesium oxide in the uidized bed 33 and serve to accumulate thereupon any stray magnetic particles or material that may be fortuitously introduced into the upper chamber of the vessel 11 along with the magnesium oxide from the hop-per 50 by the feeding mechanism 53. From time to time, when the machine 10 is shutdown, the interior of the vessel 11 may be suitably cleaned, including removing the magnetic particles from the permanent magnets 61, and including renewing of the protective liner or coating 60, in an obvious manner.

Further, the machine lll comprises a device, indicated at 100, that is employed in the loading of the sheaths of the assemblies Ifor the electric heating units of the sheathed resistance conductor type; which device is described more fully hereinafter in conjunction with FIG. 2; and which device 1610 comprises, among other elements, an inlet pipe 101 and an outlet pipe 102.

Further, the machine 10 comprises a control valve 70, including a valve casing 71, housing a rotatably mounted valve element 72. The upper portion of the upper chamber 13 disposed above the iiuidized bed 33 in the vessel 111 is connected by a first pipe 81 to the upper portion of the valve casing 71; the lower portion of the upper chamber 13 disposed below the lluidized bed 33 in the vessel 11 `is connected by a second pipe 82 tothe intermediate portion of the valve casing 71; and lower portion of the valve casing 71 is connected to a downwardly extending third pipe 33. in the arrangement, the first pipe S1 is substantially horizontally disposed adiacent to the vessel 11 and substantially vertically disposed adjacent to the valve casing 71, the second pipe 82 is upwardly inclined from the vessel 11 to the valve casing 71, and the third pipe 83 is substantially vertically disposed extending directly downwardly from the valve casing 71. The inlet conduit 101 is upwardly directed and is provided with a anged fitting 103 at the extreme upper end thereof that is connected to a complementary fitting 84 carried by the extreme lower end of the third pipe S3. The outlet conduit 102 is outwardly directed and is provided with a flanged fitting 104 at the extreme outer end thereof that is connected to a complementary hanged fitting 85 carried by the extreme outer end of a fourth pipe 86. The fourth pipe 86 includes a manually operable valve S7 and the other end thereof is connected to the inlet of a 'pump 88 that is provided with an outlet that is connected to a fifth pipe Sg, and the extreme outer end of the fifth pipe 89 communicates with the conduit 18 immediately below the lower frusto-conical section 17 of the vessel 11.

Considering now the general mode of operation of the machine 10, in conjunction with the filling or loading device 100, it is pointed out that the third pipe 83 may be completely cutoff from the first and second pipes 81 and 82 by operation of the valve element 72 of the valve 70 into its position with respect to the valve casing 71, as illustrated in FIG. 1; and likewise, the fourth pipe 86 may be closed by operation of the manual valve 87 into its closed position. After an assembly for an electric heating unit of the sheathed resistance conductor type has been placed in the loading device 100, it is desirable to purge air therefrom; which step is effected by operation of the valve 70 so that the valve element 72 is arranged to connect the first pipe 81 to the third pipe 83 and to disconnect the second pipe 82 from both of the pipes 81 and 83. At this time, argon gas from the upper portion of the upper chamber 13 above the lluidized bed 33 in the vessel 11 is conducted via the first pipe S1 through the valve 70 into the third pipe 83 and thence into the inlet conduit 81 extending to the loading device 100; whereby the argon gas supplied to the loading device 100 is utilized for the purpose of purging air from the assembly supported thereby, as explained more fully hereinafter. Thereafter, it is desirable to cause the conveying gas carrying the suspended magnesium oxide to be supplied to the loading device 100 for the loading purpose; whereby the valve 70 is operated so as to cause the valve element 72 to connect the second pipe 82 to the third pipe 33 and to disconnect the first pipe 81 from both of the pipes 82 and 83. At this time, the conveying gas suspending the magnesium oxide is delivered from the fluidized bed 33 in the vessel 11 in the lower portion of the upper chamber 13 upwardly through the second conduit 82 and through the valve 70 into the third pipe 83 and thence into the inlet conduit 101 extending to the loading device 100. During the loading operation of the loading device 100, the valve 37 in the fourth pipe 86 is operated into its open position; whereby in the loading operation, the conveying gas carrying the magnesium oxide is supplied to the inlet conduit 101 and delivered to the loading device 100, wherein the magnesium oxide is separated from the conveying gas and deposited into the sheath of the assembly for the filling purpose, and the thus stripped conveying gas is returned via the outlet conduit 102 through the valve 37 in its open position into the fourth pipe 86 and thence to the pump 83. Also, during the loading operation, the pump 88 is operated; whereby the stripped argon gas is returned from the fourth pipe S6 by the pump 88 into the fifth pipe 89 and thence into the conduit for recirculation through the vessel 11, all in the manner previously explained. After the filling or loading of the sheath of the assembly in the loading device 100, the valve 70 may be again operated so as to cutoff the connection of the third pipe 33 from both of the pipes 81 and 32; whereby any residual magnesium oxide in the second pipe 82 may slide downwardly therealong by the action of gravity back into the lower portion of the upper chamber 13. Also, at this time, the valve 87 may be closed so as to cutoff the loading device 100 from the fourth pipe 8d so as to prevent the introduction of air into the fourth pipe S5 and consequently into the vessel 11 by the pump 83 incident to the break-down of the loading device for the purpose of removing therefrom the loaded assembly for the electric heating unit following the filling of the sheath thereof, in the manner described above.

Of course, it will be understood that the loading dcvice 100 is operated in conjunction with the machine 10 repeatedly, inthe manner described above, for the purpose of loading the sheaths of successive ones of the electric heating units with the magnesium oxide in the manufacture of successive ones of the electric heating units.

Also, at this point, it is noted that only one of the loading devices 100 has been illustrated in the interest of simplicity; however, it will, of course, be appreciated that a plurality of the loading devices 100 may be utilized in conjunction with the fundamental arrangement of the machine 10, in an obvious manner; whereby in this case, the machine 10 would supply the various loading devices 100 with the magnesium oxide for the loading purpose, all in the manner previously described.

Considering now in greater detail the construction and arrangement of the loading device 100, and referring to FIG. 2, the loading device 100 further comprises an upstanding substantially cylindrical shell 111 provided with upper and lower frusto-conical end walls 112 and 113. An upstanding upper tube 114 is sealed in place in a substantially centrally disposed opening provided in the upper end wall 112; and the inner end of the previously mentioned inlet conduit 101 is sealed in an opening prof vided in the upper portion of the tube 111 adjacent to the upper end thereof; an upstanding lower tube 115 is sealed in place in a substantially centrally disposed opening provided in the lower end wall 113; and the upper end of the lower tube 115 terminates in an upwardly and outwardly liared funnel 116. The tubes 114 and 115 are arranged in alignment with each other substantially centrally of the longitudinal axis of the shell 111, and the extreme lower end of the upper tube 114 terminates within the confines of the upper end of the funnel 116 and in spaced relation with respect thereto. The extreme inner end of the outlet conduit 102 is sealed in an opening provided in the upper end wall 112.

As previously noted, the loading device 100 is adapted to receive an assembly for an electric heating unit of the sheathed resistance conductor type, indicated at 200; which assembly 200 comprises an elongated helical resistance conductor 201 electrically connected at the opposite ends thereof to upper and lower electric terminals 202 and 203, respectively, as by welding, as indicated at 202a and 2035:. Further, the assembly 200 comprises an elongated tubular metallic sheath 204 surrounding the resistance conductor 201 and at least the inner ends of the terminals 202 and 203 and spaced radially outwardly therefrom so as to define an annular void or cavity 205 therein surrounding the elements 201, 202 and 203. Further, an inwardly projecting annular ridge 204a is formed in the lower portion of the tubular sheath 204 adjacent to the extreme lower end thereof; which annular ridge 204:1 constitutes a stopper ledge that is employed in stoppering the extreme lower end of the sheath 202, as explained more fully hereinafter.

Further, the extreme upper end of the upper terminal 202 is provided with grooved structure defining a head 206 that is adapted to be supported by an upper loading hook 121 forming a part of the loading device 100; and likewise, the extreme lower end of the lower terminal 203 is provided with grooved structure defining a head 208 9 that is adapted to be supported by a lower loading hook 122 forming a part of the loading device 100.

Further, the loading device 100 comprises an upper stopper 123 that is adapted to be removably supported in place in the extreme open top end of the upper tube 114; which stopper 124 is provided with a centrally disposed opening therein in which there is arranged a cylindrical bushing 124 having a centrally disposed opening therethrough slidably receiving and supporting the upper hook 121. The extreme lower end of the tube 115 is provided with a cylindrical flexible sealing collar 125 that is adapted to envelope and to seal the extreme lower end of the lower tube 115 to the extreme upper end of the tubular sheath 204 of the assembly 200. Further, the loading device 100 comprises a supporting plate 126 that is adapted to engage the extreme lower end of the tubular sheath 204 of the assembly 200; which supporting plate 126 is provided with an opening 127 therethrough, through which the lower hook 122 is selectively movable.

Considering now the manner in which the assembly 200 for the electric heating unit is produced in conjunction with the loading device 100, the subassembly including the resistance conductor 201 and the terminals 202 and 203, as well as the tubular sheath 204 are prefabricated in any sutiable manner and brought into the vicinity of the loading device 100. The subassembly of the resistance conductor 201 and the upper and lower terminals 202 and 203 is suitably threaded through the sheath 204, the upper end of the upper terminal 202 being projected upwardly beyond the top end of the sheath 204. The upper hook 121 is projected downwardly through the bushing 124 so that the extreme lower end thereof projects downwardly below the extreme lower end of the lower tube 115, whereupon the extreme lower end of the upper hook 121 is connected to the upper head 206 provided on the extreme upper end of the upper terminal 202. The upper hook 121 is then elevated and the extreme upper end of the sheath 204 is secured by the sealing sleeve 125 to the extreme upper end of the lower tube 115. At this time, the supporting plate 126 is disposed well below the extreme lower end of the tubular sheath 204, and the lower terminal 203 is projected from the extreme lower end of the tubular sheath 204, whereupon the lower stopper 209 and the associated lock washer 210 are secured to the grooved structure provided on the lower end of the lower terminal 203 just above the lower head 200, and the lower hook 122 is then secured to the lower head 200. The lower hook 122 is then raised upwardly through the opening 127 provided in the supporting plate 12e forcing the lower stopper 209 into position in the lower end of the tubular sheath 206, but still somewhat below its ultimately seatedposition upon the annular shoulder 20411 provided in the lower portion of the tubular sheath 204; whereby the lower stopper 209 is disposed somewhat below the cooperating stoppering shoulder 204:1, so that the lower end of the sheath 204 is not completely stoppered at this time. Also, the lower supporting plate 126 is moved upwardly into engagement with the extreme lower end of `the tubular sheath 204 in supporting relation therewith; and further, the upper hook 121 is raised so as to stretch the subassembly of the elements 201, 202 and 203 substantially into its position, as illustrated in FIG. 2, and disposed out of contact with the surrounding tubular sheath 204.

At this time, it is assumed that the machine 10 is in operation, whereby the valve 70 is operated so as to effect the connection of the rst pipe 81 to the third pipe S3 and the consequent delivery of argon gas from the vessel 11 into the inlet conduit 101. Also, at this time, the valve 07 in the fourth pipe 86 occupies its closed position so as to disconnect the loading device 100 from the fourth pipe 06 in the manner previously explained. The supply of argon gas into the inlet conduit 101 at this time purges or drives the air from the loading device 100 and from the assembly 200 through the sheath 204 and out of the lower end thereof through the opening 127 in the supporting plate 126 by virtue of the fact that the lower stopper 209 does not occupy its linal stoppering position with respect to the associated stoppering shoulder 204:1. T his purging operation is continued until all of the air is purged from the loading device and from the assembly 200, whereupon the lower hook 122 is raised further into its tinal position so as to force the lower stopper 209 into its final stoppering position with respect to the cooperating stoppering shoulder 204a. At this time, the valve 70 of the machine 10 is operated to cutofrr the connection between the first pipe 81 and the third pipe 83; and the upper hook 121 is raised into its iinal position so that at this time the assembly of the elements 201, 202 and 203 occupies its final position, as illustrated in FIG. 2, with the elements 201, 202 and 203 in spaced relation with the tubular sheath 204.

The loading device 100 and the assembly 200 are now in condition for the lling of the cavity 205 within the sheath 204 with the magnesium oxide, whereupon the valve 70 of the machine 10 is operated so as to connect the second pipe 82 to the third pipe 03, in the manner previously explained, so that the argon gas carrying the suspended magnesium oxide is blown through the inlet conduit 101 into the upper end of the upper tube 114 and then blown downwardly therethrough and through the funnel 116 and the lower tube 115 into the sheath 204, As the argon gas carrying the suspended magnesium oxide is thus blown downwardly into the sheath 204, the magnesium oxide is projected further downwardly into theV bottom thereof and into the bottom of the cavity 205 upon the top of the bottom stopper 209 and separated from the conveying argon gas in the funnel 116, where- As the argon gas carrying the suspended magnesium oxide flows upwardly in the funnel 116 in counterow relation with respect to the incoming stream of argon gas carrying the magnesium oxide, whereby the thus stripped argon gas escapes from the top of the funnel 116 into the shell 111 and thence into the outlet conduit 102. Also, a substantial proportion of the argon gas initially charged into the sheath 204 ows upwardly therein, as the loading proceeds, and through the lower tube 115 and the funnel 116 into the shell 111 and thence into the outlet conduit 102. Further, at this time, the valve 8-7 in the fourth pipe 86 in the machine 10 is operated into its open position so that the stripped argon gas passes from the outlet conduit 102 through the fourth pipe 86 and thence into the pump 8S from which it is pumped via the lifth pipe 89 back into the conduit 18 for recirculation through the vessel 11, in the manner previously explained.

Accordingly, the magnesium oxide carried by the argon gas is blown through the inlet conduit 101 into the loading device 100, whereupon the magnesium oxide is stripped from the conveying argon gas, and the thus stripped conveying argon gas is returned via the outlet conduit 102 from the loading device 100 back to the vessel 11, with the result that as this step is continued, the cavity 205 in the sheath 204 is iilled from the bottom toward the top thereof in an obvious manner. As the sheath 204 is thus iilled with the granular refractory material (the crystalline magnesium oxide), it is recommended that the sheath 204 be vibrated or tapped lightly adjacent to the level of the magnesium oxide accumulating therein; and to this end, there is provided in conjunction with the loading device 100, a hammering device 150, as diagrammatically illustrated in FIG. l. More particularly, the hammering device comprises a pivotally mounted hammer 151 that is adapted to be reciprocated so as to hammer or tap the sheath 204 as the assembly 200 undergoes the above-described loading operation in the loading device 100; and as previously explained, the hammering device 150 is gradually elevated during the loading step so as to insure tamping in place of the magnesium oxide as it is deposited in the sheath 204 in the assembly 200. As the magnesium oxide is thus loaded into the sheath 204, it embeds first the lower terminal 203 and then the helical resistance conductor 201 and then the upper terminal 202; and the tamping or vibrating of the sheath 204 causes the granular magnesium oxide to fill the interior of the helix of the resistance conductor 201, so that at the conclusion of this loading step, a bed of the magnesium oxide is provided in the sheath 204 completely filling the same and all voids therein so as completely to embed the helical resistance conductor 201 and the inner ends of the terminals 201 and 203, thereby retaining these elements in place in spaced-apart relation radially inwardly with respect to the surrounding tubular sheath 204.

At the conclusion of the loading operation in the loading device 100, the valve 70 of the machine 10 is returned into its position illustrated in FIG. 1 effecting cutoff of the third pipe 83 from both of the pipes 81 and S2; and moreover, the valve 87 in the fourth pipe S6 is operated into its closed position. At this time, the loaded assembly 200 may be removed from the loading device 100 by lowering of the supporting plate 126 followed by disconnection of the lower hook 122 and followed by lowering of the sheath 204 with respect to the sealing collar 12S and the lowering of the upper hook 121 in the upper stopper 123. More particularly, the upper hook 121 is projected downwardly through the extreme lower end of the sealing collar 124 and disconnected from the upper head 206 carried by the upper terminal 202. Finally, at this time, the open upper end of the sheath 204 is suitably stoppered between the extreme upper end thereof and the upper terminal 202 projecting outwardly therefrom. Normally, the loaded assembly 200 is further processed in order to produce the finished electric heating unit; which further processing of the loaded assembly 200 ordinarily includes the step of rolling the same in a rolling machine, such, for example, as that disclosed in the previously mentioned Oakley patent. Of course, ultimately, the extreme lower end of the sheath 204 is stripped back to expose the extreme outer end of the lower terminal 203; and thereafter, the extreme outer ends of the terminals projecting from the adjacent outer ends of the sheath 204 are severed or cutoff so as to eliminate the grooved structures formed therein, including the heads 206 and 208. Ultimately, the opposite ends of the sheath 204 are suitably sealed utilizing ceramic plugs, or the like, disposed within the opposite ends of the sheath 204 and surrounding the adjacent intermediate portions of the terminals 202 and 203 respectively projecting from the opposite ends of the sheath 204, all in the usual manner.

Referring now to FIG. 3, a modified form of the loading device 300 is there illustrated that is useful, alternatively with the loading device 100, in conjunction with the machine which loading device 300 is especially useful for loading the sheath of an assembly 400 for an electric heating unit of the sheathed resistance conductor type, wherein the assembly 400 includes an exceedingly high resistance helical resistance conductor 401 formed of exceedingly flexible electrical resistance wire and respectively connected at the opposite ends thereof to the upper and lower terminals 402 and 403, in the manner previously explained. In this construction of the loading machine 300, an upstanding outer cylindrical shell 311 is provided through which the inner elongated tube 314 projects. More particularly, the elongated tube 314 projects downwardly through the shell 311 in radially inwardly spaced relation with respect to the lower tube 315 that is formed integrally with the lower end wall 313 of the shell 311. Moreover, the elongated tube 314 is movable through the upper end wall 312 of the shell 311 in guided relation with respect thereto by an associated cylindrical bushing 312a provided in a cooperating opening formed in the upper end wall 312 and arranged in sliding sealed relation with the elongated tube 314. In the loading device, 300, the inlet conduit 301 from the machine 10 communicates directly with the interior of the shell 311, while the outlet conduit 302 communicates directly with the interior of the elongated tube 314 adjacent to the extreme upper end thereof; and furthermore, the extreme upper end of the tube 311 is closed by an associated cap 323 carrying a guide bushing 324 in which the upper hook 321 is slidably arranged. The lower end of the loading machine 300 is identical to that of the loading machine and has not been illustrated in the interest of brevity.

Further considering the construction of the loading machine 300, the lower end of the lower tube 314 and the upper end of the tubular sheath 404 of the assembly 400 are removably secured together by the sealing collar 325; and in the arrangement, the elongated tube 314 initially projects downwardly through the lower tube 315 well into the sheath 404 of the supported assembly 400. Specifically, when the assembly 400 for the electric heating unit is arranged in supported position in cooperating relation with the loading device 300, the extreme lower end of the elongated tube 314 may terminate adjacent to the extreme lower end of the resistance conductor 401 and adjacent to the extreme inner end of the lower terminal 403, as illustrated in FIG. 3.

In the operation of the loading device 300, in conjunction with the machine 10, the air in the loading device 300 and in the `assembly 400 is first purged therefrom by the introduction of the argon gas from the machine 100 into the inlet conduit 301; whereby the loading device 300 and the assembly 400 are purged of the contained air in the manner previously explained.

Subsequently, in the loading of the sheath 404 of the assembly 400, the conveying argon gas carrying the magnesium oxide is blown through the inlet conduit 301 in the manner previously explained; whereby the same enters the shell 311 and proceeds downwardly therethrough and through the lower tube 315 into the sheath 404, and specifically into the annular space between the sheath 404 and the elongated tube 314, thereby to deposit the magnesium oxide in the cavity 405 in the assembly 400, in the manner previously explained. More particularly, the magnesium oxide is projected downwardly in the annular space mentioned between the tubular sheath 404 and the elongated tube 314 and is deposited in embedding relation with respect to the lower terminal 403; whereby the conveying argon gas thus stripped of the magnesium oxide enters the extreme lower end of the elongated tube 314 and passes upwardly therethrough and into the outlet conduit 302 to be returned into the machine 10, in the manner previously explained.

As the filling operation proceeds, the elongated tube 314 is elevated, so as to prevent stoppering of the extreme lower open end thereof, as the magnesium oxide is loaded into the sheath 404 of the assembly 400, and so as to protect the highly flexible resistance conductor 401 in the loading operation.

At the conclusion of the loading operation, the elongated tube 314 has been elevated relative to the shell 311 so that the extreme lower end thereof has been Withdrawn from the extreme upper end of the tubular sheath 404; and in passing, it is noted that this elev-ation of the elongated tube 314 with respect to the shell 311 and with respect to the sheath 404 of the assembly 400 is also with respect to the upper hook 312 that occupies a stationary position in the loading operation. Furthermore, it is mentioned that since the elongated tube 314 is elevated with respect to the sheath 404 of the assembly 400 in the loading operation, it is necessary to provide a flexible connection, not shown, in the outlet conduit 302 to accommodate the vertical movement of the elongated tube 314 in the loading operation and with respect to the fourth pipe 86 forming a part of the machine 10.

The subsequent steps involved in the finishing of the loaded assembly 400 are identical to those in conjunction with the finishing of the loaded assembly 200, as previously described.

As previously explained, either of the loading devices 100 or 300 may be utilized in conjunction with the ma- 13 chine 10. Ordinarily, Athe loading device ltttlt will be employed in the manufacture of relatively high wattage electric heating units due to the circumstance that the electric resistance elements thereof are formed of relatively coarse resistance wire in helical form requiring no particular support thereof in the assembly 200 during the loading operation. On the other hand, the loading device 300 may be advantageously employed in the manufacture of relatively low wattage electric heating units due to the circumstance that the electric resistance elements thereof are formed of relatively fine resistance wire in helical form requiring some support thereof in the assembly 400 during the loading operation. Of course, the last-mentioned support of the resistance conductor or element 40'1 of the assembly 400 as illustrated in FIG. 3, is provided by the surrounding position of the elongated tube 314 of the loading device 300 in the loading operation as previously described.

Reconsidering the operations of the loading devices 100 and 300, it will be appreciated that not only are the sheaths 2,04 and 494 of the respective assemblies 290 and 466 loaded with the charges of granular refractory material, as described above, but also the respective sheaths 204 and 404 thereof are filled with the argon gas in the interstices or minute voids between the individual particles of granular material in the respective charges. Accordingly, in each of the finished electric heating units the corresponding sheath thereof is filled with the composite charge of granular refractory material and argon gas; which arrangement is very advantageous in the iinished electric heating unit by virtue of the f-act that the argon gas is highly inert, thereby preventing subsequent oxidation of the electric heating element and other internal p-arts of the finished electric heating unit. Of course, it will be understood that this result is automatically achieved in carrying out the present method. rl`his result is impossible to obtain, utilizing conventional methods in which the sheath of the electric heating unit is filled with the granular material in the presence of air, since it will be apparent that it is impossible to eliminate the occluded air from the charge of granular material in an electric heating unit that is made in accordance with conventional methods.

ln view of the foregoing, it is apparent that there has been provided an improved method of loading with granular refractory material the sheath of an assembly for an electric heating unit of the sheathed resistance conductor type that involves a blowing step, wherein the granular refractory material is introduced into the sheath of the assembly mentioned by a stream of conveying gas and then separated therefrom with the escape of the thus stripped conveying gas from the sheath of the assembly mentioned; whereby the introduction and loading of the sheath of the assembly produces a simple and economical manner of effecting the required embedding of the resistance conductor and the inner ends of the terminals of the assembly, with the resulting economy in the rapid and economical manufacture of the electric heating units.

While there has been described what is at present considered to be the preferred embodiment of the invention, it will be understood that various modifications may be made therein, and it is intended to cover in the appended claims all such modifications -as fall within the true spirit and scope of the invention.

What is claimed is:

1. The method of making an electric heating unit comprising providing an assembly of an elongated electric resistance conductor and a pair of electric terminals respectively electrically connected to the opposite ends of said resistance conductor and an elongated tubular metallic sheath surrounding said resistance conductor and at least the inner ends of said terminals, arranging said assembly in upstanding position with said resistance conductor and the inner ends of said terminals in spaced relation with said sheath and with the upper end of said sheath in open condition and with the lower end of said sheath in closed condition, blowing chemically inert gas having granular refractory material suspended therein in a first stream downwardly into the open upper end of said sheath, stripping the granular refractory material from the first stream and projecting the same downwardly through said sheath, conducting the thus stripped gas in a second stream upwardly through said sheath and out of the open upper end thereof, and continuing said three steps last set forth until a filling of the granular refractory material in said sheath is produced that embeds said resistance conductor and the inner ends of said terminals and that retains the same in place in spaced relation with said sheath.

2. The method of making an electric heating unit comprisin y providing an assembly of an elongated electric resistance conductor and a pair of electric terminals respectively electrically connected to the opposite ends of said resistance conductor and an elongated tubular metallic sheath surrounding said resistance conductor and at least the inner ends of said terminals, arranging said assembly in upstanding position with said resistance conductor and the inner ends of said terminals in spaced relation with said sheath and with the upper end of said sheath in open condition and with the lower end of said sheath in closed condition, producing a predetermined suspension of granular refractory material in a body of chemically inert gas eX- teriorly of said sheath, blowing the gas having the predetermined suspension of the granular refractory material therein from the body in a first stream downwardly into the open upper end of said sheath, stripping the granular refractory material from the first stream and projecting the same downwardly through said sheath, conducting the thus stripped gas in a second stream upwardly through said sheath and out of the open upper end thereof, returning the second stream back to said body, and continuing said iive steps last set forth until a filling of the granular refractory material in said sheath is produced that embeds said resistance conductor and the inner ends of said terminals and that retains the same in place in spaced relation with said sheath.

3. The method of making an electric heating unit comprising providing an assembly of an elongated electric resistance conductor and a pair of electric terminals respectively electrically connected to the opposite ends of said resistance conductor and an elongated tubular metallic sheath surrounding said resistance conductor and at least the inner ends of said terminals, arranging said assembly in upstanding position with said resistance conductor and the inner ends of said terminals in spaced relation with said sheath and with the upper end of said sheath in open condition and with the lower end of said sheath in closed condition, fluidizing a bed of granular refractory material with a body of circulated chemically inert gas exteriorly of said sheath, blowing the gas having the granular refractory material suspended therein from said fluidized bed in a first stream downwardly into the open upper end of said sheath, stripping the granular refractory material from the first stream and projecting the same downwardly through said sheath, conducting the thus stripped gas in a second stream upwardly through said sheath and out of the open upper end thereof, returning the second stream back to said body to be recirculated through said uidized bed, and continuing said five steps last set forth until a filling of the granular refractory material accumulates in said sheath embedding said resistance conductor and the inner ends of said terminals and retaining the same in place in spaced relation with said sheath.

4. The method set forth in claim 1, wherein the granular refractory material mentioned consists essentially of magnesium oxide.

5. The method set forth in claim 1, wherein the chemically inert gas mentioned consists essentially of argon.

6. The method of making an electric heating unit comprising providing an assembly of an elongated electric resistance conductor and a pair of electric terminals respectively electrically connected to the opposite ends of said resistance conductor and an elongated tubular metallic sheath surrounding said resistance conductor and at least the inner ends of said terminals and a tube arranged within said sheath and surrounded thereby and surrounding said resistance conductor, arranging said assembly in upstanding position with the upper end of said sheath in open condition and with the lower end of said sheath in closed condition and with both ends of said tube in open condition and with the open lower end of said tube in communication with the closed lower end of said sheath, blowing chemically inert gas having granular refractory material suspended therein in a first stream downwardly into the open upper end of said sheath, stripping the granular refractory material from the first stream in said sheath and adjacent to the open lower end of said tube and projecting the same downwardly through said sheath, conducting the thus stripped gas from said sheath into the lower open end of said tube and thence in a second stream upwardly therethrough and then out of the open upper end thereof, and continuing said three steps last set forth while progressively elevating said tube in said assembly and with respect to said resistance conductor and said sheath and until a lilling of the granular refractory material in said sheath is produced that embeds Said resistance conductor and the inner ends of said terminals and that retains the same in place in spaced relation with said sheath.

7. The method of making an electric heating unit comprising providing an assembly of an elongated electric resistance conductor and a pair of electric terminals respectively electrically connected to the opposite ends of said resistance conductor and an elongated tubular metallic sheath surrounding said resistance conductor and at least the inner ends of said terminals, arranging said assembly 3 in upstanding position with said resistance conductor and the inner ends of said terminals in spaced relation with said sheath and with the upper and lower ends of said sheath in open condition, blowing chemically inert gas into the open upper end of said sheath and downwardly in a first stream therethrough and out of the open lower end thereof for a time interval sufficiently long -to purge from said sheath substantially all ofthe air contained therein and to substitute therefor a lling of the gas, then closing the open lower end of said sheath, then blowing the gas having granular refractory material suspended therein in a second stream downwardly into the open upper end of said sheath, stripping the granular refractory material from the second stream and projecting the same downwardly through said sheath, conducting the thus stripped gas in a third stream upwardly through said sheath and out of the open upper end thereof, and continuing said three steps last set forth until a filling of the granular refractory material in said sheath is produced that embeds said resistance conductor and the inner ends of said terminals and that retains the same in place in spaced relation with said sheath.

References Cited by the Examiner UNITED STATES PATENTS 1,859,562 5/32 Huston 141-67 2,376,840 5/45 Wiegand 53-22 2,490,934 12/49 Vogel 29-155.64 2,596,135 5/52 Dowling et al 141-67 2,636,642 4/53 Gorin 222-195 2,677,172 5/54 Oakley 29-155.64 2,955,796 10/60 Titchenal 141-68 3,040,490 6/ 62 Virta 53-28 5 JOHN F. CAMPBELL, Primary Examiner.

WHITMORE A. WILTZ, Examiner. 

1. THE METHOD OF MAKING AN ELECTRIC HEATING UNIT COMPRISING PROVIDING AN ASSEMBLY OF AN ELONGATED ELECTRIC RESISTANCE CONDUCTOR AND A PAIR OF ELECTRIC TERMINALS RESPECTIVELY ELECTRICALLY CONNECTED TO THE OPPOSITE ENDS OF SAID RESISTANCE CONDUCTOR AND AN ELONGATED TUBULAR METALLIC SHEATH SURROUNDING SAID RESISTANCE CONDUCTOR AND AT LEAST THE INNER ENDS OF SAID TERMINALS, ARRANGING SAID ASSEMBLY IN UPSTANDING POSITION WITH SAID RESISTANCE CONDUCTOR AND THE INNER ENDS OF SAID TERMINALS IN SPACED RELATION WITH SAID SHEATH AND WITH THE UPPER END OF SAID SHEATH IN OPEN CONDITION AND WITH THE LOWER END OF SAID SHEATH IN CLOSED CONDITION, BLOWING CHEMICALLY INERT GAS HAVING GRANULAR REFRACTORY MATERIAL SUSPENDED THEREIN IN A FIRST STREAM DOWNWARDLY INTO THE OPEN UPPER END OF SAID SHEATH, STRIPPING THE GRANULAR REFRACTORY MATERIAL FROM THE FIRST STREAM AND PROJECTING THE SAME DOWNWARDLY THROUGH SAID SHEATH, CONDUCTING THE THUS STRIPPED GAS IN A SECOND STREAM UPWARDLY THROUGH SAID SHEATH AND OUT OF THE OPEN UPPER END THEREOF, AND CONTINUING SAID THREE STEPS LAST SET FORTH UNTIL A FILLING OF THE GRANULAR REFRACTORY MATERIAL IN SAID SHEATH IS PRODUCED THAT EMBEDS SAID RESISTANCE CONDUCTOR AND THE INNER ENDS OF SAID TERMINALS AND THAT RETAINS THE SAME IN SPACED RELATION WITH SAID SHEATH. 